1. Field of Invention
The present invention relates to a rocking follower mechanism for a three-dimensional cam. In particular, the invention relates to a rocking follower mechanism for a three dimensional cam for transmitting a positional variation of a cam surface of a three-dimensional cam to a valve lifter, in response to revolutions of an internal combustion engine.
2. Description of Related Art
There is a known variable valve timing mechanism capable of varying the on-off timing of an intake valve or an exhaust valve of an internal combustion engine. This is performed in accordance with operation of an internal combustion engine. In one of such variable valve timing mechanisms, there is a known mechanism as shown in FIG. 10. A lift amount of a valve 103 is varied using a three-dimensional cam 102 movable in the rotational axial direction so as to adjust the on-off timing of the valve, as disclosed in Japanese Patent Application Laid-open No. Hei 10-196333, for example.
In such a variable valve timing mechanism using a three-dimensional cam, a tilt angle of a cam surface 102a varies with the rotation. Also, a guide groove 105, extending in parallel with the rotational direction of the three-dimensional cam 102, is formed in the top surface 104a of a valve lifter 104. A semi-columnar follower 106 is capable of rocking in accordance with variation in the tilt angle of the cam surface 102a. The follower 106 is disposed in the guide groove 105 such that the three-dimensional cam 102 is sufficiently brought into contact with the valve lifter 104. This results in enhanced durability.
Further, in such a structure, the cam surface 102a of the three dimensional cam 102 slides on a cam sliding surface of the semi-columnar follower 106 in the axial direction thereof. Therefore, as shown in FIG. 10, the semi-columnar follower 106 has a wide portion 106b formed at its center. The guide groove 105 also has a wide groove 105a formed therein into which the wide portion 106b is inserted. The above described structure may allow a thrust surface 106c of the wide portion 106b to abut against a thrust surface 105b of the expanded- width groove 105a. As a result, the axial movement of the follower 106 is suppressed against the sliding movement of the cam surface 102a.
However, the wide portion 106b, formed in the center of the follower 106, is required to have a cam sliding surface 106d. The cam sliding surface 106d radially extends from the cam sliding surface 106a of the follower 106, on which the three-dimensional cam 102 slides.
The cam surface 102a of the three-dimensional cam 102 varies its position in contact with the cam sliding surface 106a, of the follower 106, by moving along a shaft 107 axially. As a result, the lift amount of the valve 103 is varied. Therefore, a width Cw of the cam surface 102a, in the axial direction, is greater than a width Fw of the cam sliding surface 106a of the follower 106.
Further, the sliding position between the cam surface 102a, of the three-dimensional cam 102, and the sliding surface 106a, of the follower 106, always varies in the axial direction of the follower 106 (in the direction of the arrow Z in FIG. 10). This variance is in response to the rotation of the three-dimensional cam 102.
Therefore, the cam surface 102a, of the three dimensional cam 102, slides so as to move along a portion defined by the cam sliding surface 106a, that is not adjacent to the cam sliding surface 106d of the wide portion 106b, and the cam sliding surface 106a, which is adjacent to the cam sliding surface 106d. If the sliding position is moved, the cam surface 102a of the three-dimensional cam 102 collides against an angular portion 106e. The angular portion 106e is defined by the thrust surface 106c and the cam sliding surface 106d of the wide portion 106b.
The aforementioned collision is likely to generate a hit sound. As may be appreciated, this sound is not preferable in view of driving environment of a motor vehicle, for example. Further, the collision may cause abrasion on the cam surface 102a, of the three-dimensional cam 102, as well as the cam sliding surface 106d of the wide portion 106b. This abrasion is heavy in comparison with the abrasion caused by the normal sliding movement. Accordingly, such abrasion resulting from the collision is not preferable in view of the durability of the variable valve timing mechanism.